Viral infections in mammals, and especially in humans, are very widespread and in spite of the considerable progress made in general chemotherapy, little progress has been made in creating specific drugs which either cure viral diseases or alleviate symptoms of patients afflicted with these diseases.
The use of various nucleoside analogs as agents for the treatment of cancer, fungal infections, bacterial infections and viral infections is not new. In the treatment of viral infections, the treatment of Herpes Simplex Virus (HSV), related Herpes infections and Human Immunodeficiency Virus (HIV) with nucleoside analogs is now part of the armamentarium of the medical practitioner.
Unlike the design of anti-cancer or anti-bacterial agents, the design of antiviral agents is generally more difficult. This difficulty is due to the absence of clearcut qualitative differences in the biochemistry of infected and host cells. Because viral infection results in the takeover of the replicative mechanism of infected host cells, the targeting of DNA and its constituent bases as a means of antiviral agent design has been pursued with some success. Various purine and pyrimidine analogs have been synthesized in pursuit of more effective chemotherapeutic agents. In particular, thymidine, a base exclusive to DNA, has become a primary target for structural modification to obtain selective antiviral agents.
A number of pyrimidine nucleoside analogs have shown significant activity as antiviral agents, particularly against the herpes group of viruses. The herpes viruses that infect humans comprise 5 different groups and include Herpes Simplex Virus Types I and II (HSV I and II), which are responsible for herpes labialis, herpes keratitis, herpes encephalitis and herpes genitalis, Varicella Zoster (VZV), which is responsible for chicken pox and shingles, Cytomegalovirus (CMV), which is responsible for neonatal disease and Epstein-Barr Virus (EBV), which is responsible for infectious mononucleosis and Burkitt's lymphoma.
Herpes viruses have been known to cause cancer in animals and there is sufficient evidence suggesting a close relationship between these viral infections and human malignancies (See, for example, Shugar, FEBS Lett., 40, S48, 1974; Cancer Research, 34, 1083, 1974; and Federation Proc., Part I, 31, 1625, 1974). Consequently, certain antiviral agents have direct usefulness in antitumor chemotherapy.
Viral infection manifests itself by overtaking the host cell's replication mechanism. Consequently, there exist certain enzymes which are necessary for elaborating the virus via this mechanism. Selective antiviral agents may be designed to target a unique property of the virus-induced enzyme which is not shared by the corresponding host enzyme. Two of these target enzymes in the herpes group of viruses include virus encoded thymidine kinase and DNA-polymerase (Davis, et al., J. Virol., 13, 140, 1974). In those viruses which produce Acquired Immunodeficiency Syndrome (AIDS), the viral replication mechanism often proceeds through an RNA dependent DNA polymerase or Reverse Transcriptase, which may serve as a potential target for anti-AIDS compounds.
The first of the above-mentioned enzymatic targets, viral-induced thymidine kinase, is responsible for the phosphorylation of deoxythymidine to deoxythymidine monophosphate (dTMP). This virus-induced enzyme is synthesized in the infected cell, upon infection with a virus. However, the host cell generally also contains its own thymidine kinase, usually of two major types; cytoplasmic and mitochondrial.
The second potential enzyme target includes the Viral-induced DNA polymerase(s), which are responsible for the biosynthesis of viral DNA. This enzyme(s) functions to replicate the virus and induce the various enzymes that are responsible for viral function.
These two viral enzymes are useful targets for antiviral agents. An agent that could target the viral-induced thymidine kinase or DNA polymerase(s) without interfering with the human thymidine kinase would be a potential therapeutic agent for viral infections.
Certain nucleoside analogs have been shown to be useful as antiviral agents, but also have evidenced certain limitations. For example, 5-substituted deoxyuridine analogs, such as 5-ethyl, 5-propyl and 5-allyl deoxyuridine derivatives have been shown to be alternate substrates for thymidine kinase and have shown selected affinity for the viral-induced thymidine kinase (Cheng, et al., Antimicrob. Agents and Chemotherapy, 10, 119, 1976). However, these nucleoside derivatives were found to be substrates for the host enzyme, human thymidine phosphorylase, one of the enzymes which is responsible for thymidine degradation.
Other nucleoside analogs that have shown to be effective antiviral agents against HSV and VZV include acyclovir (ACV), bromovinyl deoxyuridine (BVDU) and 2,-fluoro-5-iodo-arabinosylcytosine (FIAC) (Schaeffer, et al., Nature, 272, 583 1978; Elion, et al., Proc. Nat. Acad. Sci. USA. 74, 5716, 1977; De Clercq, et al., Proc. Nat. Acad. Sci. USA,, 76, 2947, 1979; Watanabe, et al., J. Med. Chem., 22, 21, 1979; Fox, et al., Antiviral Chemo.. 219, 1981; Davis, et al., J. Virol., 26, 603, 1978; Allaudeen, et al., Proc. Nat. Acad. Sci. USA, 78, 2698, 1981; and De Clerq, E., Pure and Appl. Chem., 55, 623 1983). These compounds are selectively phosphorylated by viral thymidine kinase and their phosphorylated forms inhibit the viral DNA polymerase.
Another viral disease which recently has been studied greatly and treated with only limited success is AIDS. AIDS is a generally fatal disease caused by a human pathogenic retrovirus known as human T-lymphotropic virus type III (HTLV III), lymphadenopathy-associated virus (LAV) or human immunodeficiency virus (HIV). (Barre-Sinoussi, et al., Science, 220, 868, 1983 and Mitsuya, et al., Proc. Nat. Acad. Sci. USA, 82, 7096, 1985).
In comparison with the other T-lymphotropic retroviruses HTLV I and II, HTLV III (HIV) and lymphoadenopathy viruses are nontransforming cytopathic viruses without immortalizing activity. The viral replication process is believed to be an important event in the progress of AIDS. It is further believed that the enzyme Reverse Transcriptase plays an essential role in the elaboration and life cycle of HIV and consequently, the progress of the disease. It is therefore believed that this enzyme may be a particularly appropriate target for the development of potential drugs against AIDs because of the absence of such an enzyme in the uninfected host cell.
Recently, investigators have studied a number of antiviral agents as potential anti-AIDS agents, e.g., ribavirin (See, for example, McCormick, et al., Lancet. ii. 1367, 1984; Gilbert and Knight, Antimicrobial Agents and Chemotherapy, 30, 201, 1986; and Robins, et al., Adv. Enzyme Regul., 24, 29, 1986) and Suramin (Mitsuya, at al., Science. 226, 172, 1984), among others. A number of nucleosides have played important roles in the treatment of RNA and DNA viral diseases. 3'-azido-3'deoxythymidine (AZT) and 2',3'-Dideoxynucleosides, for example, 2',3'-dideoxycytidine (DDC) and 2',3'-dideoxyadenosine and 2',3'-dideoxyinosine, among other nucleosides, have shown promise as potential anti-AIDS agents (Richman, et al., N. Engl. J. Med., 317, 192 1987; and Mitsuya, Proc. Nat. Acad. Sci. USA. 83, 1911, 1986). Certain of these 2',3'-dideoxynucleosides of cytidine and adenosine in the form of their respective 5'-triphosphate derivatives can act as chain terminators because of their lack of a 3'-hydroxyl group for forming a phosphotriester linkage. Other thymidine and cytidine derivatives such as 3'-amino, 3'-azido and 2',3'L- dideoxy-2',3'-didehydro analogues have also exhibited anti-HIV activity (Lin and Prusoff, J. Med. Chem.. 21, 109, 1978; Balzarini, et al., Mol. Pharmacol., 32, 162, 1987; Kim, et al., J. Med. Chem., 30, 862, 1987; Baba, et al., Biochem. Biophys. Res. Comm., 142, 128, 1987; Lin, et al., J. Med. Chem., 30, 440, 1987; and Lin, et al., Biochem. pharmacol., 36, 311, 1987).
Certain biological characteristics of nucleoside analogs in general may limit their use as anti-viral agents. These characteristics include their toxicity, metabolic inactivation (by for example, cytidine deamination and thymidine and uridine phosphorylation) and lack of selectivity (Holy, Nucleosides and Nucleotides, 81, 147 , 1978). In certain cases, these agents may be incorporated into DNA, resulting in teratogenicity and mutagenicity (Renis, Antiobiotics Chemother., 27, 164, 1980).
Metabolic inactivation is a common mechanism by which certain nucleosides are limited in therapeutic value. For example, although anti-viral activity has been demonstrated by a number of 2'-deoxyuridine derivatives containing 5-substituents such as 5-iodo, bromo, ethynyl, propyl, trifluoromethyl, ethyl, S-methyl, nitro, cyanato, iodovinyl, and bromovinyl, among others. (See, for example, Rapp and Vanderslice, Virology, 22, 321, 1964; Rawls, et al., Proc. Soc. Exp. Biol. Med., 115, 123, 1964; Prusoff and Ward, Biochem. Pharmacol., 25, 1233, 1976; Salzman, Virology, 10, 150, 1960; Kaufman and Heidelberger, Science, 145, 585, 1964; Heidelberger and Boohar, Biochem. Biophys. Acta, 91, 639, 1964; De Clercq, et al., Molec. Pharmacol., 14, 422, 1978; Nemes and Hilleman, Proc. Soc. Exp. Biol. Med., 119, 515, 1965; De Clercq and Shugar, Biochem. Pharmacol., 24, 1073, 1975; Gupta, et al., J. Med. Chem., 18, 973, 1975; De Clercq, et al., Antimicrob. Agents Chemother., 13, 545, 1978; Cheng, Biochim. Biophys. Acta, 452, 370, 1976; Hardi, et al., Antimicro. Agents Chemother., 10, 682, 1976; Kotick, et al., J. Org. Chem., 34, 3806, 1969; Ryu and Bardos, J. Heterocyc. Chem.. 16, 1049, 1979; Torrence, et al., J. Med. Chem., 20, 974, 1977; Bardos and Kalman, J. Pharm. Sci., 55, 606, 1966; and De Clercq, et al., J. Med. Chem., 26, 661, I983), their corresponding cytidine analogs undergo the above-mentioned enzymatic deamination to give compounds with less or no activity, this cripples their biological utility. Those degradation products which are thymidine analogs further limits their utility because they are generally substrates for thymidine phosphorylase (metabolic inactivation) (Schroeder, et al., J. Med. Chem., 24, 109, 1981).
The antiviral activity of certain of these nucleosides is believed to be due, at least in part, to their conversion into triphosphates followed by their incorporation into DNA (See, Stellwagen and Tompkins, Proc. Natl. Acad. Sci. USA, 68, 1147, 1971; Stellwagen and Tompkins, J. Mol. Biol., 56, 167, 1971; Graham and Whitmore, Cancer Res., 30, 2636, 1970; and Dannenberg and Heidelberger, J. Med. Chem., 16, 712, 1973). This, unfortunately often produces mutagenic effects in the host organism, especially for such analogues as 5-iodo-2'-deoxyuridine. However, such mutagenic effects are minimal for certain analogues which exhibit base-pairing properties in DNA which are similar for those of the parent base. This is true for 5-ethyl, propyl and SCH.sub.3 derivatives of 2'-deoxyuridine. In particular, 5-ethyl-2'-deoxyuridine has been found to have significant activity against herpes simplex and vaccinia viruses, comparable to that of 5-iodo-2'-deoxyuridine, but without the concomitant mutagenicity (Bernaerts and De Clercq, Nucleosides and Nucleotides, 6, 421, 1987; Kulikowski, and Shugar, J. Med. Chem., 17, 269, 1974; and Swierkowski and Shugar, J. Med. Chem., 12, 533, 1969).
A number of 5-substituted 2-pyrimidinone-2'-deoxynucleoside analogs, including methyl-, fluoro-, iodo-, bromo-, ethynyl and propynyl- have also been shown to exhibit antiviral activity, apparently by inhibiting virus specific thymidine kinase (Efange, et al., J. Med. Chem., 28, 904, 1985 and Lewandowski, et al., Antimicrob. Agents and Chemother., 33, 340, 1989).
Several modifications on the sugars of nucleosides have produced agents having good antiviral activity. Several arabinosides, including ara-C and Ara-T have shown good antiviral activity (Chen, et al., J. Biol. Chem., 251, 4833, 1976; Neenan and Rohde, J. Med. Chem., 16, 580, 1973; and Cheng, et al., Ann. N.Y. Acad. Sci., 255, 332, 1975). Substitutions at the 5'- position with amino, azido groups have rendered highly selective antiherpes agents, whereas substitution at the 3'- position with an azido group has provided potent anti-HIV activity. Several 2',3'-modified nucleosides have been studied for antiherpes activity and several 2',3'-dideoxy-2',3'-didehydro and dideoxy compounds have shown activity against a variety of viruses, particularly HIV.
Very little work has been done on 5-substituted 2-pyrazinone N-oxide nucleoside analogs, which is the subject of the present application. The riboside of 2-pyrazinone-4-N-oxide has been found to have antibacterial activity (Bobek and Bloch, J. Med. Chem., 15, 164, 1972). Its 2'-deoxyribonucleoside proved to be significantly more effective as an antimicrobial agent than the riboside, but was essentially inactive against leukemic L1210 cells (Berkowitz, et al., J. Med. Chem., 16, 183, 1973). In contrast, the 2'-deoxyribonucleoside of 5-methyl-2-pyrazinone-4-N-oxide was found to be significantly more active against leukemic L1210 cells and only marginally active against the bacterial cells (Bobek and Bloch, J. Med. Chem., 20, 458, 1977). None of these agents have been shown to be effective as antiviral agents.